One kind of a mass flow meter that is known is a thermal mass flow meter comprising a main flow channel where a sample gas flows, a sensor flow channel that divides the sample gas by being bifurcated from the main flow channel and that is provided with a flow rate detecting mechanism to detect a mass flow rate of the sample gas and a bypass flow channel that is arranged between a bifurcating point and a meeting point in the sensor flow channel. This kind of the thermal mass flow meter has a flow rate detecting mechanism comprising an upstream side sensor section and a downstream side sensor section each being formed by winding two thermosensitive resistive bodies in a shape of a coil around an outer side of a hollow narrow pipe made of metal forming the sensor flow channel and a bridge circuit arranged for each sensor section.
More concretely, the hollow narrow pipe is heated by the thermosensitive resistive bodies and a temperature distribution is symmetric to an axis of a center of the hollow narrow pipe while the sample gas does not flow. Meanwhile, the temperature at the downstream side sensor section becomes higher than the temperature at the upstream side sensor because the sample gas heated by the upstream side sensor flows into the downstream side sensor section while the sample gas flows so that the temperature difference is formed between the upstream side sensor section and the downstream side sensor. As a result, the temperature distribution becomes asymmetric.
Since a certain relationship is formed between the temperature difference (ΔT) and the mass flow rate of the sample gas, the mass flow rate can be measured by detecting the temperature difference by means of the bridge circuit (for example, refer to the patent document 1).
However, the above-mentioned mass flow meter has a problem that the measurement accuracy of the flow rate drops because of the error in the measured flow rate in a case where the gas supply pressure (primary side pressure) is changed (for example, 100 kPa) in the main flow channel where the mass flow meter is arranged.
In addition, there is a problem that the error of the measured flow rate resulting from the change of the primary side pressure varies depending on a kind of the sample gas.    Patent document 1: Japan patent laid-open number 7-271447